Thin film transistors (TFTs) are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. For applications in which a transistor needs to be applied to a substrate, a thin film transistor is typically used. A critical step in fabricating the thin film transistor involves the deposition of a semiconductor onto the substrate. Presently, most thin film devices are made using vacuum deposited amorphous silicon as the semiconductor.
Amorphous silicon as a semiconductor for use in TFTs still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively difficult or complicated processes such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow deposition on substrates made of certain plastics that might otherwise be desirable for use in applications such as flexible displays. Furthermore, vacuum deposition of semiconductors typically continuously cover a substrate, requiring subsequent subtractive patterning steps. These patterning steps are time consuming and waste materials.
There is a growing interest in depositing thin film semiconductors on plastic or flexible substrates, particularly because these supports would be more mechanically robust, lighter weight, and potentially lead to cheaper manufacturing by allowing roll-to-roll processing. A useful example of a flexible substrate is polyethylene terephthalate. Such plastics, however, limit device processing to below 200° C.
In the past decade, various materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of thin film transistors. Semiconductor materials are desirable that are simpler to process, especially those that are soluble in organic or aqueous solvents and, therefore, capable of being applied to large areas by relatively simple processes, such as spin coating, dip coating, microcontact printing, or ink jet application. Semiconductor materials that can be deposited at lower temperatures would open up a wider range of substrate materials, including plastics, for flexible electronic devices. Furthermore, additive solution processes have the opportunity to reduce materials cost by only applying semiconductor materials where they are needed.
Thus, thin film transistors made of coatable semiconductor materials can be viewed as a potential key technology for circuitry in various electronic devices or components such as display backplanes, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations.
This, in turn, has fueled an interest in discovering new semiconductor materials. Organic semiconductors comprise one broad class of low-temperature alternative semiconductor materials that have been the subject of considerable research efforts. However, most organic semiconductors generally have inferior or problematic electronic properties compared to amorphous silicon for use in transistor devices. For example, organic materials may tend to degrade in normal atmospheric conditions. In contrast, inorganic semiconductors tend to be more stable. Consequently, an inorganic semiconductor that is compatible with temperature-sensitive substrates and that has electronic properties equivalent to amorphous silicon would enable electronics for a variety of flexible substrates.
The discovery of new inorganic semiconductors has also been the subject of considerable research efforts. For example, metal oxide semiconductors are known that constitute zinc oxide, indium oxide, tin oxide, or cadmium oxide deposited with or without additional doping elements including transition metals such as aluminum. Such semiconductor materials, which are transparent, can have an additional advantage for certain applications, as discussed below.
Various processes for making zinc oxide films have been disclosed, both high temperature and low temperature processes, including radio-frequency magnetron sputtering or modified reactive planar magnetron sputtering.
Ohya et al (Japanese Journal of Applied Physics, Part 1, January 2001, vol. 40, no. 1, pages 297-8 disclose a thin film transistor of ZnO fabricated by chemical solution deposition.
Transparent conducting oxides are reviewed in the August 2000 issue of the Materials Research Bulletin, Volume 25 (8) 2000, devoted to materials and properties of transparent conducting oxide compounds.
One low temperature process for deposition of such oxide semiconductors is disclosed in US 2004/0127038 A1 to Carcia et al. This patent discloses a semiconductor deposition process that uses magnetron sputtering of a metal oxide (ZnO, In2O3, SnO2, CdO) or metal (Zn, In, Sn, Cd) target in an atmosphere with a controlled partial pressure of oxygen in an inert gas. This is a low temperature process that is compatible with temperature sensitive substrates and components, for example, drive circuits for displays on flexible, polymer substrates. The field effect transistors of Carcia et al. are based on a nominally undoped metal oxide semiconductor that must be deposited using physical vapor deposition or chemical vapor deposition, preferably rf (radio frequency) magnetron sputtering.
Japanese Kokai JP2004349583 A1 discloses a method of producing a thin film transistor in which a dispersion of zinc-oxide nanoparticles is ink-jetted to form the semiconducting channel. No actual examples, however, are described relating to the preparation of the dispersion.
US 2004/0127038 discloses a method to produce high quality zinc-oxide thin film transistors using sputtering as a vacuum deposition method.
Steven K. Volkman et al., “A novel transparent air-stable printable n-type semiconductor technology using ZnO nanoparticles,” 2004 IEEE International Electron Device meeting Technical Digest, pp. 769, 2004, discloses a method for producing thin film transistors using organically stabilized zinc-oxide nanoparticles. The disclosed process involves an exposure to a temperature of 400° C. or plasma hydrogenation.
Transparent oxide semiconductors are especially useful in the fabrication of transparent thin film transistors. Such transparent transistors can be used to control pixels in a display. By being transparent, the active area of the transistor can be significantly increased.
For example, thin film transistors are employed in active-matrix liquid crystal displays (AMLCD), which are extensively used in laptop computers and other information display products. The operation of an AMLCD display requires that each picture or display element (pixel) have a corresponding thin film transistor associated with it for selecting or addressing the pixel to be on or off (“pixel driver”). Presently, AMLCD displays employ transistor materials that may be deposited onto glass substrates but are not transparent (typically amorphous, polycrystalline, or continuous-grain silicon deposited on glass). The portion of the display glass occupied by the addressing electronics is not available for transmission of light through the display. Transparent transistors for AMLCD addressing would allow greater light transmission through the display, thereby improving display performance.
Semiconductor materials for use in thin film transistors in various electronic devices may require significant mobilities, well above 0.01 cm2/Vs, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000.
Semiconductor materials include “p-type” or “p-channel” semiconductors, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device. An alternative to p-type semiconductor materials are “n-type” or “n-channel” semiconductor materials, which terminology indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device. Thus, in an n-type TFT, the device can be turned on, by applying a more positive voltage.